I believe you still have to stop at certain points to check the responses. You might not want to send a message if one of the recipients is invalid, but only that command will return an error code, and the message sending will succeed.

You'd also need to prevent command injection. If the response code to DATA is an error, but you sent the message anyway, the whole message body will be interpreted as commands. Oops! The line ending bug (SMTP smuggling discovered early this year) was bad enough.